Corrosion inhibitors and synergistic inhibitor combinations for the protection of light metals in heat-transfer fluids and engine coolants

ABSTRACT

Corrosion protection of magnesium and aluminum alloys in engine coolants and heat-exchange fluids is achieved by the use of a select group of aliphatic and aromatic carboxylate acids or the alkaline metal, ammonium or amine salts thereof in combination with a fluoride and/or a fluorocarboxylic acid or salt thereof. These compositions have been found to significantly improve the high temperature magnesium corrosion protection properties of coolants, and are of use in automobile engine coolant systems.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to compositions which assist in thecorrosion protection of magnesium, magnesium-aluminum,aluminum-magnesium and aluminum alloys in engine coolants andheat-exchange fluids. Aliphatic mono- or di-basic acids or aromaticcarboxylate acids, or the alkali metal, ammonium or amine salts thereof,have been found to provide corrosion protection to magnesium, inaddition to the protection that these acids already provide to othermetals, such as aluminum, iron, copper and solder, when combined with afluoride or a fluorocarboxylic acid or a salt thereof. Specificcombinations of such acids or salts with fluoro compounds have beenfound to provide synergistic corrosion protection for magnesium. Theoptional addition of a hydrocarbyl triazole and/or a thiazole to thesecombinations provides improved corrosion protection, particularly tocopper alloys but also to other metals such as aluminum. The presence ofthe fluoride and/or a fluorocarboxylate has been found to significantlyimprove the high temperature magnesium corrosion protection propertiesfor individual carboxylates, for combinations of carboxylates, and forcombinations of carboxylates and hydrocarbyl triazoles and/or thiazoles.

[0003] Automobile engine cooling systems contain a variety of metals,including copper, solder, brass, steel, cast iron, aluminum, magnesium,and their alloys. The possibility of corrosive attack on such metals ishigh, due to the presence of various ions as well as the hightemperatures, pressures, and flow rates found in such cooling systems.The presence of corrosion products within the cooling system caninterfere with heat transfer from the engine combustion chambers, whichmay subsequently cause engine overheating and engine component failuredue to excess metal temperatures. See generally Fay, “Antifreezes andDeicing Fluids,” Kirk-Othmer Encyclopaedia of Chemical Technology (1978)ed, vol 3, pp 79-95. It would therefore be generally advantageous if theformation of corrosion products within automobile cooling systems couldbe controlled or eliminated. It is one object of the instant inventionto provide a corrosion inhibitor useful in the prevention and control ofcorrosion in automobile engine cooling systems containing variousmetals, particularly magnesium.

[0004] The trend towards improved fuel economy for automobiles has ledto the increased use of lightweight materials such as aluminum andmagnesium alloys for engine and cooling system components. However, ithas been found that pitting and crevice corrosion are particularlyprevalent in aluminum and magnesium-containing cooling systems. Pittingof thin-walled automobile radiator tubes may lead to tube perforation.Crevice corrosion at cylinder head packings or coolant hose connectionsmay also occur. Both types of corrosion may lead to eventual coolantloss, with subsequent engine overheating and component failure. Otherforms of localized corrosion such as deposit attack from deposition ofcorrosion products may also result.

[0005] Many conventional corrosion inhibitor additives used inautomobile cooling systems do not provide adequate protection againstthe pitting, crevice, and deposit attack corrosion phenomena found withmagnesium, aluminum and various other metal alloys. It would thereforebe particularly advantageous if such localized corrosion phenomena couldbe controlled or eliminated. It is another object of the instantinvention to provide a corrosion inhibitor for use in automobile coolingsystems, which prevents or controls localized corrosion of magnesium.

[0006] All corrosion inhibitors employed in automobileantifreeze/coolant formulations are gradually depleted by use and thebuild-up of corrosion products in the cooling system. It would thus beadvantageous if the build-up of corrosion products within the system andsubsequent corrosion inhibitor depletion or degradation could becontrolled or eliminated. It is a further object of the instantinvention to provide a corrosion inhibitor which is less prone todepletion or degradation than traditional corrosion inhibitors used inantifreeze/coolant formulations.

DESCRIPTION OF RELATED INFORMATION

[0007] Organic Acid Technology (OAT) coolants and heat exchange fluidshave been introduced, providing improved corrosion protection and havinga long life. OAT corrosion-inhibitor packages in aqueous and glycolconcentrates are used in automotive, heavy duty, marine and industrialapplications. OAT corrosion-inhibitors are also used in secondarycooling systems and in a variety of industrial heat exchange fluids.Several U.S. and foreign patent references disclose the use ofcarboxylic acids, or the salts of such acids as corrosion inhibitors inantifreeze/coolant and heat-exchange fluid compositions. Thesecompositions are optimized for the protection of aluminum and othermaterials currently used in the above applications.

[0008] Various corrosion inhibitors have been added to heat transferfluids to reduce corrosion of metallic systems. For example, U.S. Pat.No. 4,587,028 (Darden) discloses non-silicate antifreeze formulationscontaining alkali metal salts of benzoic acid, dicarboxylic acid andnitrate. Additional ingredients including alkali metal hydroxides,alkali metal nitrates and aromatic triazoles, such as tolyltriazole orbenzotriazole are preferably provided. U.S. Pat. No. 4,647,392 (Dardenet al) discloses corrosion inhibitors using aliphatic monobasic acids orsalts, hydrocarbyl dibasic acids or salts and hydrocarbonyl triazole.U.S. Pat. No 4,657,689 (Darden) discloses corrosion inhibitorscontaining aliphatic monobasic acids or salts, hydrocarbyl dibasic acidsor salts, hydrocarbyl azoles and specific hydrocarbyl alkali metalsulfonates. U.S. Pat. No. 5,085,791 (Burns) discloses antifreezecompositions containing cyclohexane acid corrosion inhibitor alone or incombination with other corrosion inhibitors, particularly sebacic acidand tolyltriazole. The cyclohexane acid includes cyclohexyl carboxylic(formic) acid, cyclohexyl acetic acid and cyclohexyl propionic acid. Thecyclohexane acid is targeted to inhibit lead solder and/or aluminumcorrosion. U.S. Pat. No. 4,105,405 (Wehle et al) discloses the use ofcyclohexane hexacarboxylic acid corrosion inhibitors.

[0009] JP-A-08 020763 (Seiken Kogaku Kogyo KK) describes a fluid for useas a coolant in internal combustion engines comprising glycol, water, amagnesium compound, an alkyl benzoic acid (e.g. p-tert-butylbenzoicacid) or a salt, a do-decane diacid or a salt and a triazole orthiazole.

[0010] JP-A-08 085782 (Nippon Chem Kogyo KK) describes a glycol basedantifreeze composition containing a dodecanedioic acid or a salt,p-tert-butylbenzoic acid or a salt and a triazole, together with asilicate, molybdate, benzoate or thiazole. The composition is free fromamines, phosphates, borates and nitrites.

[0011] Engine manufacturers are now evaluating the use of magnesium as amaterial for engine and heat-transfer systems. Traditional inhibitorpackages do not give adequate corrosion protection to magnesiumcomponents. In general, currently used OAT coolants are only mildlyaggressive, but the protection levels for magnesium are not sufficient,particularly at the high temperatures found in working engines. Previousresearch has indicated that a combination of an alkylbenzoic acid(4-tert-butylbenzoic acid), an aliphatic monoacid (octanoic acid) and ahydrocarbyl triazole (tolyltriazole), provides improved corrosionprotection for magnesium, in comparison with traditional and OAT coolantformulations (Table 1 in U.S. Pat. No. 4,851,145 (van Neste)).EP-A-0229254 (Asahi Glass Co Ltd) describes an electrolytic capacitorcomprising a capacitor element and an electrolyte impregnated into theelement, wherein the electrolyte contains a fluorocarboxylic acid orsalt dissolved in an organic solvent.

[0012] There is need for a coolant system which provides high levels ofcorrosion protection to magnesium components.

SUMMARY OF THE INVENTION

[0013] The present invention relates to an antifreeze concentratecomposition comprising one or more inhibitors selected from alkylbenzoicacid, C₅-C₁₅ mono- or di-basic acid or salts thereof, together with afluoride and/or a fluorocarboxylic acid or salt thereof.

[0014] Amongst the aromatic carboxylates, the group of alkylbenzoicacids of general formula:

R—Ar—COOH

[0015] where Ar is benzyl and R is a C₁-C₈ alkyl radical or an elementof group 7, e.g. F, are preferred. 4-Tert-butylbenzoic acid (hereafterreferred to as PTBBA), is the most preferred alkylbenzoic acid and4-fluoro benzoic acid is the most preferred compound when R is anelement of group 7. Alkali metal, ammonium or amine salts of thealkylbenzoic acid may be used. The most preferred alkali metal salts arepotassium and sodium.

[0016] Aliphatic carboxylate acids that provide corrosion protection maybe any C₅-C₁₅ aliphatic monobasic acid or dibasic acid or the alkalimetal, ammonium or amine salt of said acids. Heptanoic acid, octanoicacid, and nonanoic acid (or isomers thereof), and mixtures thereof arethe preferred monobasic acids. Decanoic acid and undecanoic acid providegood protection but solubility of the higher alkyl acids in water islimited. Of the dibasic acids, dodecanedioic acid provides reasonablygood magnesium protection. Of the naphtylcarboxylic acids,1-naphtylcarboxylic acid is the preferred carboxylic acid.

[0017] It has been found that the combination of one or more of theabove described acids, together with the fluoro compound, gives asynergetic effect for improved magnesium protection. The combination ofPTBBA and octanoic acid is especially preferred. Nonanoic and heptanoicacid are good alternatives for octanoic acid. Compositions formulatedwith no hydrocarbyl triazole give good magnesium protection, orhydrocarbyl triazole at a level below 0.2 wt %. The addition of ahydrocarbyl triazole to these combinations provides additional copperprotection, as expected. Improved corrosion protection properties werefound for the other metals, especially for aluminum.

[0018] It has been found that the addition of a fluoride or a fluorocarboxylate to an aliphatic carboxylic acid or to an alkylbenzoic acidor to the preferred synergistic combinations of alkylbenzoic acids andthe preferred monocarboxylic acids, significantly improves the hightemperature corrosion protection properties of the formulation. Mostparticularly, such systems give improved protection against magnesiumcorrosion.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention firstly identifies a group of specificaliphatic and aromatic carboxylates that provide improved corrosionprotection, when used with a fluoride and/or a fluorocarboxylic acid orsalt thereof.

[0020] Amongst the aromatic carboxylates are preferred the group ofalkylbenzoic acids of general formula:

R—Ar—COOH

[0021] where Ar is benzyl and R is a C₁-C₈ alkyl radical. In thisinvention, 4-tert-butylbenzoic acid (PTBBA) is preferred. Any alkalimetal, ammonium or amine salts of the alkylbenzoic acid may be used toform the alkylbenzoic acid salt; however, alkali metals are preferred.The most preferred alkali metal salts are potassium and sodium. Of thenaphtylcarboxylic acids, 1-naphtylcarboxylic acid is the preferredcarboxylic acid.

[0022] Aliphatic carboxylate acids that provide corrosion protection maybe any C₅-C₁₅ aliphatic monobasic acid or dibasic acid or the alkalimetal, ammonium or amine salt of said acids. This would include one ormore of the following acids or isomers thereof: heptanoic acid, octanoicacid, and nonanoic acid, and mixtures thereof are the preferredmonobasic acids. Decanoic acid and undecanoic acid provide goodprotection but solubility of the higher alkyl acids in water is limited.Of the dibasic acids, dodecanedioic acid provides reasonably goodmagnesium protection.

[0023] Combinations of one or multiples of the above mentioned aromaticcarboxylates with one or more aliphatic carboxylates, together with thefluoro compound, give a synergistic effect for the protection ofmagnesium alloys. When using a hydrocarbyl triazole component such as anaromatic triazole or an alkyl substituted aromatic triazole such asbenzotriazole or tolyltriazole, increased protection of copper isachieved. Improved protection of other metals, particularly aluminum isalso observed. For magnesium protection alone the use of a hydrocarbyltriazole is optional.

[0024] The additional inhibitor which now provides superior synergisticcorrosion protection, especially for magnesium alloys, especially atelevated temperatures, in combination with one carboxylic acid or with asynergetic mixtures of multiple acids such as the combination ofaliphatic carboxylates with the alkylbenzoic acids, such as mentionedabove, is fluoride. Used alone, fluoride provides a very low level ofprotection. It is an important aspect of this invention that fluorcompounds such as fluorides and/or fluoro carboxylates, in combinationwith aliphatic carboxylates or alkylbenzoic acids give a synergisticeffect to significantly reduce corrosion, especially of magnesium atelevated temperatures. The fluoride can be introduced in the formulationas hydrogen fluoride and/or a fluoro carboxylic acid or the alkalimetal, ammonium or amine soluble salt of the mentioned acid.

[0025] The combination of the corrosion inhibitors of the instantinvention will most typically be employed in antifreeze formulations asa coolant for internal combustion engines. Other applications mayinclude hydraulic fluids, aqueous cutting oils, paints, soluble oils,metal cutting fluids, aircraft deicers, and greases. In theseapplications the aliphatic acids and alkylbenzoates may be formed withmetal hydroxides including sodium, potassium, lithium and barium.

[0026] In a preferred embodiment of the instant invention, the abovedescribed corrosion inhibitors may be employed in a mixture with aliquid alcohol freezing point depressant to form a novelantifreeze/coolant concentrate composition for use in the coolingsystems of internal combustion engines. The coolant concentratecomposition preferably comprises: from 80-99 weight percent of a watersoluble liquid alcohol freezing point depressant; from 0.01-15,preferably 0.1-5 weight percent of the above described alkylbenzoicacid/salt and/or aliphatic carboxylic acid/salt component and from0.005-5, preferably 0.01-1 weight percent of hydrogen fluoride and/or afluoro carboxylate. In addition one of the above mentioned components,hydrocarbyl triazole components can optionally be used at from 0.005-1,preferably 0.1-0.3 weight percent.

[0027] The liquid alcohol freezing point depressant component of theabove described coolant in the instant invention includes glycols suchas ethylene glycol, di-ethylene glycol, propylene glycol, di-propyleneglycol and glycol monoethers such as the methyl, ethyl, propyl and butylethers of ethylene glycol, di-ethylene glycol, propylene glycol anddi-propylene glycol. Ethylene and propylene glycol are particularlypreferred as the freezing point depressant component. In the abovedescribed coolant concentrate composition of the instant invention, anadditional dicarboxylic acid preferably dodecanoic diacid, may beemployed at concentrations of 0.01-15.0 weight percent, preferably0.1-5.0 weight percent. Additional conventional corrosion inhibitorssuch as alkali metal borates, silicates, benzoates, nitrates, nitrites,molybdates or hydrocarbyl thiazoles may also be employed atconcentrations of 0.01-5.0 weight percent.

[0028] In another embodiment of the invention, the above describedcorrosion inhibited coolant concentrate composition is diluted with10-90 volume percent, preferably 25-75 volume percent of water. In yetanother embodiment of the invention, the above described corrosioninhibitor combinations are used in aqueous solutions of alternativefreezing point depressants such as organic acid salt solutions.Acetates, formates and proprionates are particularly preferred.

EXAMPLES

[0029] The method of the invention will be further illustrated by thefollowing examples, which are not intended to limit the scope of theinvention.

Comparative Examples Example A. (Comparative Example)

[0030] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 3 weight percent 2-ethyl hexanoic acid (2-EHA).

Example B. (Comparative Example)

[0031] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 0.2 weight percent tolyltriazole (TTZ).

Example C. (Comparative Example)

[0032] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 0.2 weight percent Hydrogen fluoride (HF) (50 weightpercent in water).

Example D. (Comparative Example)

[0033] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 3 weight percent heptanoic acid (HA).

Example E. (Comparative Example)

[0034] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 3 weight percent octanoic acid (OA).

Example F. (Comparative Example)

[0035] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 3 weight percent dodecanedioic acid (DDA).

Example G. (Comparative Example)

[0036] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 3 weight percent sebacic acid (SA).

Example H. (Comparative Example)

[0037] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 3 weight percent octanoic acid, 0.2 percent TTZ.

Example I. (Comparative Example)

[0038] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 3 weight percent PTBBA, 0.2 percent TTZ.

Example J. (Comparative Example)

[0039] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 1.5 weight percent PTBBA, 1.5 weight percentoctanoic acid, 0.2 weight percent TTZ.

Example K. (Comparative Example)

[0040] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 1.5 weight percent PTBBA, 1.5 weight percentoctanoic acid, 0.02 weight percent TTZ.

Example L. (Comparative Example)

[0041] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 1.5 weight percent PTBBA, 1.5 weight percentoctanoic acid.

Example M. (Comparative Example)

[0042] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 1.5 weight percent PTBBA, 1.5 weight percentoctanoic acid, 0.5 percent 2-ethylhexanoic acid, 0.2 weight percent TTZ.

Example N. (Comparative Example)

[0043] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 1.5 weight percent PTBBA, 1.5 weight percentoctanoic acid, 0.1 weight percent sodium metasilicate pentahydrate, 0.2weight percent TTZ.

Examples of the Invention Example 1

[0044] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 1.5 weight percent PTBBA, 1.5 weight percent OA, 0.2weight percent HF (50 weight percent in water), 0.2 weight percent TTZ.

Example 2

[0045] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 3 weight percent PTBBA, 0.2 weight percent HF.

Example 3

[0046] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 3 weight percent OA, 0.2 weight percent HF.

Example 4

[0047] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 3 weight percent heptanoic acid, 0.2 weight percentHF.

Example 5

[0048] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 1.5 weight percent PTBBA, 1.5 weight fluorobenzoicacid.

Example 6

[0049] An antifreeze formulation was prepared comprising a major amountof ethylene glycol, 1.5 weight percent PTBBA, 1.5 weight percent OA, 0.2weight percent sodium fluoride, 0.2 weight percent TTZ.

[0050] The additive systems are summarized in Table 1. TABLE 1 Summaryof Additives Alkyl Hydro benzoic carbyl Monoacids Diacids acid triazoleSilicate Fluor compound 2- Fluoro EHA HA OA SA DDA PTBBA TTZ HFcarboxylate Comparative Examples A.   3% B. 0.2% C. 0.2% D. 3% E.   3%F. 3% G. 3% H.   3% 0.2% I.   3% 0.2% J. 1.5% 1.5% 0.2% K. 1.5% 1.5%0.02%  L. 1.5% 1.5% M. 0.5% 1.5% 1.5% 0.2% N. 1.5% 1.5% 0.2% 0.1%Invention Examples 1 1.5% 1.5% 0.2% 0.2% 2   3% 0.2% 3   3% 0.2% 4 3%0.2% 5 1.5% 1.5% 6 1.5% 1.5% 0.2% 0.2%

[0051] Invention Example 1 describes the combination of the addition ofa fluor compound to a combination of alkylbenzoic acid andmonobasic/dibasic acid. Invention Example 2 describes the combination ofthe addition of a fluor compound to an alkylbenzoic acid. InventionExamples 3, 4 and 5 describe the combination of the addition of a fluorcompound to an aliphatic carboxylic acid. The fluor compound can be afluoride (3 and 4) or a fluoro-carboxylate (5). Invention Example 6 issimilar to Invention Example 1, but sodium fluoride is used instead ofhydrogen fluoride.

[0052] It is well known that corrosion resistance of a metal or metalalloy depends upon both the stability of its passivating oxideprotective film and its ability to repassivate active corrosion regionson the surface of the metal or alloy. The speed of corrosion on theother hand is related to the current density. A rapid CyclicPolarization Scanning (RCP) technique, based on cyclic potentiokineticpolarization and described in the CEBELCOR (Centre Beige d'Etude de lacorrosion) publication Rapports Techniques, Vol. 147, R. T. 272 (August1984), may be used to determine the susceptibility of a metal or alloyto localized corrosion. The RCP technique measures rupture or pittingpotential (E_(r)), repassivation potential (E_(p)) and current density(I_(a)) for a given metal or alloy. E_(r) is the potential at which thepassivating film of a given material breaks down, and is directlyrelated to the pitting tendency of the material in a particularenvironment. E_(p) is the potential at which active corrosion regions ofthe material are repassivated in a given environment. I_(a) is thecurrent density which is a measure for the general corrosion rate. Thehigher I_(a), the higher the corrosion rate. E_(r), E_(p) and I_(a) aremeasured with a silver reference electrode and a working electrodeconstructed from the material subject to corrosive attack. The higher(more positive) the E_(r) value, the more effective a given antifreezeformulation is in preventing pitting corrosion initiation and progress.Similarly, a higher (more positive) E_(p) value indicates that theparticular corrosion inhibitor formulation has a greater ability torepassivate existing pits and crevices. On the other hand the higherI_(a) value the faster corrosion is proceeding, the less effective aparticular corrosion inhibitor is for protecting a certain metal oralloy.

[0053] The RCP test procedure may be described as follows; polishedspecimens of the metal to be tested (working electrode) are immersed ina 30 percent by volume solution of a given antifreeze concentrateformulation in hard ASTM corrosive water; that is water containing 148mg/l sodium sulfate, 165 mg/l sodium chloride, 138 mg/l sodiumbicarbonate (ASTM 1384 corrosive water) and in addition 364 mg/l calciumchloride mono hydrate.

[0054] Polarization is achieved by polarizing at a scan rate of 2mV/second until rupture potential E_(r) is attained. A rapid increase inpolarization current results at E_(r) as the protective passivating filmbreaks down. When the current reaches a predetermined maximum value, thescanning direction is reversed towards more cathodic potentials. Therepassivation potential E_(p) is determined during this final phase ofthe RCP scan.

[0055] Table 2 shows the results of RCP measurements using thecompositions of the invention on magnesium, aluminum and copper;measuring E_(r), E_(p) and I_(a). Adequate reduced corrosion rates arefound for the invention examples (1 to 6). TABLE 2 RCP measurements todetermine corrosion inhibitor effectiveness (E_(r), E_(p) in mV, I_(a)in μA/cm²) Magnesium Aluminum Copper E_(r) E_(p) I_(a) E_(r) E_(p) I_(a)E_(r) E_(p) I_(a) Comparative Examples A. −1130 * 90 −260 * 35 830 −1020 B. −1120 * 100 −400 −520 25 640 60 0.9 C. 750 * 1000 −500 −750 20 120100 20 D. −240 * 100 1390 460 50 900 −10 3 E. >2400 * 100 >2400 * 20 F.−440 * 100 960 * 30 920 3 G. −1100 * 100 340 * 40 900 6 H. >2400 −220 401420 580 30 I. >2400 * 35 −340 −520 20 J. >2400 −1120 20 >2400 * 20 17501500 1 K. >2400 * 30 >2400 * 15 1600 1200 2 L. 2200 700 30 >2400 * 151300 1000 1 M. >2400 780 30 >2400 * 20 1320 1040 2 N. 2300 −7030 >2400 >2400 20 1810 1320 2 Invention Examples 1 >2400 −1000 20 2700 * 30 300 −700 10 3 >2400 700 20 1400 * 10 4 200 * 100 1020 * 30 5300 * 30 200 * 20

[0056] The ASTM D4340 test is a standard test method which covers alaboratory screening procedure for evaluating the effectiveness ofengine coolants in combatting corrosion of aluminum casting alloys underheat-transfer conditions that may be present in aluminum cylinder headengines. For the purposes of the present application, magnesium alloywas used instead of aluminum casting alloys. A heat flux is establishedthrough a cast magnesium alloy typical of that used in engine cylinderheads whilst exposed to an engine coolant under a pressure of 193 kPa.The temperature of the magnesium specimen is maintained at 135° C. andthe test is continued for one week. The effectiveness of the coolant forpreventing corrosion of the magnesium under heat-transfer conditions isevaluated on the basis of the weight change of the test specimen andalso on the visual appearance. A heat-transfer corrosion cell isassembled according to the test methodology. The magnesium specimen is6.5 cm in diameter, and 1.3 cm thick. The system is heated through athermocouple inserted into a thermocouple hole in the test specimen. Thetest is carried out based upon 165 mg of radiant grade sodium chloridedissolved in 750 ml distilled or de-ionized water. 250 ml of the testcoolant is then added. This amount is sufficient for two tests. Theresults of the ASTM D 4340 tests are shown in FIG. 1. FIG. 1 showsphotographs of the corroded magnesium specimens after test for severalComparative Examples and much less corroded specimens for the InventionExamples (1 to 5).

[0057] It has been found that corrosion protection at high temperatures,as evaluated in the ASTM D4340 test on magnesium, is furthersignificantly improved by the synergistic effect of a fluor compound,preferably a fluoride and/or a fluoro-carboxylate. Photographs ofmagnesium coupons are shown in FIG. 1. Comparative Example (C) showsvery severe attack with deep pitting and deposits. Examples H, I and Jshow various gradations of minor attack and some deposits. The additionof a fluor compound in Examples 1 to 6, shows a dramatic improvement ofcorrosion protection and a significant reduction in deposit formation.

1. A corrosion-inhibitor formulation comprising 1) from 0.1 to 15 weight percent of one or more inhibitors selected from the group comprising an alkylbenzoic acid, a C₅-C₁₅ monobasic acid and a C₅-C₁₅ dibasic acid, or salts thereof; and 2) from 0.005 to 5 weight percent of a fluoride and/or a fluorocarboxylic acid, or salts thereof.
 2. A corrosion-inhibitor formulation as claimed in claim 1, wherein the alkylbenzoic acid is 4-tertbutylbenzoic acid.
 3. A corrosion-inhibitor formulation as claimed in claim 1, wherein said C₅-C₁₅ aliphatic monobasic acid is selected from the group consisting of octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, 2-ethylhexanoic acid and neodecanoic acid.
 4. A corrosion-inhibitor formulation as claimed in claim 3, wherein said aliphatic monobasic acid is octanoic acid.
 5. A corrosion-inhibitor formulation as claimed in claim 1, wherein said C₅-C₁₅ aliphatic dibasic acid is selected from the group consisting of suberic acid, azelaic acid, sebacic acid, undecanedioic acid, -dodecanedioic acid, and the diacid of dicyclopentadiene.
 6. A corrosion-inhibitor formulation as claimed in claim 1 further comprising a hydrocarbyl triazole or a thiazole.
 7. A corrosion-inhibitor formulation as claimed in claim 6, wherein said hydrocarbyl triazole or thiazole is present at from 0.005 to 1.0 weight percent.
 8. A corrosion-inhibitor formulation as claimed in claim 6, wherein said hydrocarbyl triazole is tolyltriazole or benzotriazole.
 9. A corrosion-inhibitor formulation as claimed in claim 1, wherein the fluorocarboxylic acid is added as the alkali metal, ammonium or amine soluble salt of the acid.
 10. A corrosion-inhibitor formulation as claimed in claim 1 further comprising an additional corrosion inhibitor selected from the group comprising alkali metal borates, alkali metal silicates, alkali metal benzoates, alkali metal nitrates, alkali metal nitrites, and alkali metal molybdates.
 11. A corrosion-inhibitor formulation as claimed in claim 1 for use with a heat transfer fluid comprising a liquid alcohol freezing point depressant.
 12. A corrosion-inhibitor formulation as claimed in claim 11, wherein said liquid alcohol freezing point depressant is ethylene glycol.
 13. A corrosion-inhibiting coolant comprising a corrosion-inhibitor as claimed in claim 1, a liquid alcohol freezing point depressant, and water.
 14. A coolant as claimed in claim 13, comprising from 25-75 volume percent water.
 15. A heat transfer fluid formulation comprising: (a) from 65-99 weight percent of a water soluble liquid alcohol freezing point depressant; (b) from 0.1-15 weight percent of alkylbenzoic acid of general formula R—Ar—COOH where Ar is benzyl and R is a C₁-C₈ alkyl radical or an element of Group 7, or a salt thereof; (c) from 0.1-15 weight percent of C₅-C₁₅ aliphatic monobasic or dibasic acid or salt thereof; (d) from 0.01-5 weight percent of hydrogen fluoride and/or a fluorocarboxylic acid or salt thereof.
 16. The use of a corrosion-inhibitor formulation as claimed in claim 1 as a magnesium and/or aluminum corrosion-inhibitor in heat transfer fluids and engine coolants.
 17. A process for inhibiting the general pitting, crevice and deposit-attack corrosion of magnesium and/or aluminum present in the cooling system of an internal combustion engine which comprises intermittently contacting the metal surface comprising magnesium and/or aluminum to be inhibited against corrosion with a heat transfer fluid comprising a corrosion-inhibitor formulation as defined in claim
 1. 